Scarlett Johansson onstage at the Marvel Studios' "Thor" and "Captain America: The First Avenger" panel. Fans whooped for Johannson, who made an unannounced Comic-Con appearance along with her "Iron Man" costar Robert Downey Jr. on Saturday.

Scarlett Johansson onstage at the Marvel Studios' "Thor" and "Captain America: The First Avenger" panel. Fans whooped for Johannson, who made an unannounced Comic-Con appearance along with her "Iron Man" costar Robert Downey Jr. on Saturday. (Kevin Winter / Getty Images)